1. Field of the Invention
The process forming the subject of the invention relates to the decarburization of chromium-containing or nickel chromium-containing pig iron containing by weight from about 1.5 to 8% of carbon, from 10 to 30% of Cr, up to 30% of Ni, and optionally Co, Mn and Mo.
2. Description of the Prior Art
Numerous processes are known for carrying out the decarburization of pig iron by the action of oxygen alone or mixed with other gases at atmospheric pressure or under a reduced pressure. The oxygen or the gaseous mixture can be placed in contact with the liquid metal, for example by injection through the bottom of a converter or, on the other hand, can be brought to the surface above the level of the metal.
In particular, in the Linz-Donawitz (LD) process, the pig iron to be decarburized is treated in a vertical converter by means of a lance pipe arranged above the level of the liquid pig iron. This lance pipe delivers a jet of oxygen which strikes the surface of the liquid metal bath.
Recent research into this process permits better understanding of the action of the jet of oxygen on the metal bath and the slag covering it.
Thus, the article of J. Schoop, W. Resch and G. Mahn entitled "Reactions Occuring During the Oxygen Top Blown Process and Calculation of Metallurgical Control Parameters", Ironmaking and Steelmaking, 1978, Vol. 5, No. 2, pages 72-79, describes the mechanism involved in the dephosphorization and the decarburization of pig iron by the LD process applied to a 200 T converter. This article shows that, in this process, the reactions between the oxygen and the liquid metal take place mainly due to the presence of droplets of liquid metal in the slag. The flow-rate of liquid metal sprayed in droplets through the slag depends on the force of impact of the jet of oxygen on the liquid metal. This flow-rate of metal can attain and even exceed one ton per second. Under these conditions, the contact surfaces between the liquid metal and the slag are multiplied by 100. A true emulsion is formed between the liquid metal, the slag and the gaseous mixture, the volume of which depends not only on the impact force of the oxygen jet but also on the characteristics of fluidity of the slag. According to this article, the phosphorus is removed preferentially in the case of low impact forces, whereas the carbon is removed preferentially in the case of high impact forces.
Analysis of the results has shown that, under conditions which are favorable to dephosphorization, the P content of the droplets is 100 times smaller than that of the metal bath. An increase in the impact force of the oxygen jet on the metal bath promotes the decarburization reaction as it causes an increase in the flow rate of sprayed droplets which can thus exceed one ton per second, as mentioned above. The very rapid decarburization now taking place is assisted by the fact that the metal droplets burst due to the formation of CO bubbles.
The article by A. Chatterjee, N. O. Lindfors and J. A. Wester entitled "Process Metallurgy of LD Steelmaking" Ironmaking and Steelmaking, 1976, Vol. 3, No. 1, describes, more particularly the procedure for decarburizing pig iron by the LD process. It shows clearly that the oxygen jet, which is supersonic at the tuyere outlet, produces by its impact an emulsion between the liquid metal, the slag and a very significant gaseous phase containing the oxygen and the carbon oxides in variable proportions. The volume of the emulsion depends largely on the viscosity of the slag. The very fluid slags rich in FeO result in the formation of emulsions with volumes which are, at the end of the oxygen blast, from three to four times that of the liquid metal. The decarburization of the droplets of liquid metal within the emulsion is caused by two concurrent processes: the oxidation of the carbon by the oxygen contained in the gaseous phase and the oxidation of the carbon by the FeO contained in the slag.
This prior art process, developed initially for the decarburization of ordinary pig iron, has been adapted for the treatment of chromium pig iron, for example in the manner described in the article by Carlson and Shaw entitled "Stainless Steel by BOF Process", Iron and Steel Engineer, August, 1972, pages 53-58. This article shows that a synthetic Cr pig iron obtained by mixing ordinary pig steel and carburized ferrochromium containing approximately 4% of carbon and approximately 15 to 16% of chromium is decarburized by insufflation of oxygen to a final C content of 0.05%. At the end of decarburization, the temperature exceeds 1,900.degree. C. In this process, mainly at the beginning of blasting of oxygen, significant quantities of Cr and Fe oxides are formed by the action of the oxygen on the pig iron and pass into the slag. Once the concentration of these oxides in the slag is sufficiently high, they react with the carbon contained in the metal bath and the CO formed in liberated. A proportion of the chromium oxide formed at the beginning of the reaction is entrained by the hot gases in the form of dust. Another proportion remains in the slag and can be reduced and recovered during a subsequent reduction operation by silicothermia.
This is therefore a process comprising several stages which demands relatively expensive retreatment of the slag in order to recover a proportion of the chromium and, moreover, it is difficult to recover the chromium oxide entrained in the hot gases. In addition, the necessary presence in this process of a slag rich in chromium oxide for effecting decarburization does not afford only advantages. In fact, there are also disadvantages since this slag reduces the effectiveness of the impact of the oxygen jet on the metal bath and therefore slows down the stirring thereof. Decarburization is therefore decelerated and, on the other hand, the losses of Cr due to oxidation increase.